Boundary conditions in the tutorial model (Periodic boundary conditions)

Hi all,

I have question regarding the boundary conditions used in the tutorial model about periodic boundary conditions. In this model a plane wave with a wave-vector with non-zero x- and z-component and polarisation in y-direction should be modelled. At the top and bottom boundary scattering boundary conditions are used in order to allow an excitation. And for the boundaries with a normal vector in x-direction Floquet periodic condition are used. But for the ones with a normal vector in y-direction perfect elector conductor is used. Can someone explain to me, while for these boundaries not also periodic conditions are used (Floquet or Continuity (as ky=0))?

(I tried to make the simulation using periodic conditions for all sides and the results are more or less the same. I'm just wondering if there is any reason why in the tutorial model perfect electric conductor is used, which may be important in some more complex geometries.)

Thanks a lot in advance,
Felix